As a light emitting type electronic display device, available are electroluminescent displays (hereinafter referred to as ELD). ELD constituting elements include inorganic electroluminescent elements (hereinafter also referred to as inorganic EL elements) and organic electroluminescent elements (hereinafter also referred to as organic EL). The inorganic EL elements have been employed as a flat type light source, and high alternating-current voltage is needed to drive them.
On the other hand, the organic electroluminescent elements emit light (fluorescence and phosphorescence) as follows. They are constituted in such a manner that a light emitting layer, which incorporates light emitting compounds, is sandwiched between a cathode and an anode, and electrons and positive holes are injected into the light emitting layer, wherein excitons are generated via their recombination, and when the resulting excitons are deactivated, light is emitted. In view of capability of light emission at a few to several ten volts, wide viewing angle due to a self light emitting type, high visibility, and space saving and portability due to a complete thin layer type solid element, organic electroluminescent elements have received attention.
Further, in recent years, in the organic electroluminescent elements, development of luminescent materials which enable preparation of organic electroluminescent element of a higher phosphorescence luminance has been diligently carried out (refer, for example, to Patent Documents 1 and 2). In conventional luminescent materials, light is emitted from an excited singlet and is called as fluorescence. Since the formation ratio of singlet excitons to triplet excitons is 1:3, the formation probability of light emitting excited species is 25%. On the other hand, in the case of luminescent materials utilizing the light emitted from the excited triplet and is called as phosphorescence, the upper limit of the internal quantum efficiency reaches 100% via internal conversion from the singlet excitons to the triplet excitons, whereby, compared to the case of fluorescence emitting materials, the maximum light emitting efficiency becomes, in principle, four times as great.
Further, the major feature of organic electroluminescent elements is in the form of a surface light source differing from conventionally employed main light sources such as a light emitting diode or a cold-cathode tube. Possible applications, which utilize the above characteristic, include light sources for lighting and backlight of various displays. It is appropriate to employ them as a backlight of liquid crystal full color displays, of which demand is markedly increasing over recent years.
When organic electroluminescent elements are employed as the above lighting source or the display backlight, they are employed as a light source which realizes white or electric bulb color (hereinafter together referred to as white). In order to realize white light emission employing organic electroluminescent elements, there available are: a method which realizes white via color mixture by regulating a plurality of light emitting materials differing in their emitted light wavelength in a single element; a method which realizes white in such a manner that for example, three colors of blue, green, and red are separately painted and they are simultaneously emitted while being mixed; and a method which realize white employing color conversion dyes (for example, a combination of a blue light emitting material and a color conversion fluorescent dye).
However, when consideration is made based on various demands such as lower cost, higher productivity, or more convenient driving methods which are demanded for the backlight, the method which realizes white via color mixture by regulating a plurality of light emitting materials which differ in emitted light wavelengths in a single element, is useful for these applications, and in recent years, research and development of the same have been increasingly conducted.
The method which realizes white based on the above method will further be detailed. There are listed: a method which realize white in such a manner that two different color light emitting materials, such as a blue light emitting material and a yellow light emitting material which result in a complementary color and the colors are mixed, and a method which realizes white in such a manner that light emitting materials of three color of blue, green, and red are employed and the colors are mixed. For example, disclosed is a method in which doping is carried out employing three phosphors, blue, green and red, as a light emitting material (for example, Patent documents 2 and 3).
Further, there is a system in which in organic electroluminescent elements which result in white light emission, each of the layers which differ in emitted light color is not in the form of an individual layer, but at least two color light emitting materials are made to coexist in a single layer, and two color lights are emitted via an energy transfer from a light emitting dopant with high light emitting energy to a light emitting dopant with a relatively low efficiency. The above method is one of the promising methods to prepare white light emitting organic EL elements since via them, it is possible to reduce the number of organic layers and to decrease the employed amount of light emitting materials. An organic electric field light emitting element is disclosed which is characterized in that a red light emitting layer and a blue light emitting layer are sequentially arranged from the anode, and the red light emitting layer incorporates at least one green color light emitting dopant (refer, for example, to Patent Document 4).
In view of color rendering and the color reproduction range, it is preferable that in electroluminescent elements which realize white light emission, at least three color light emitting materials are combined. However, since blending at least three types of light emitting materials into a single layer results in energy transfer to longer wavelength light emitting materials at a low energy level, it is very difficult to regulate the mixing ratio of these light emitting materials, resulting in fluctuations of the resulting performance. Consequently, it is typical that layers which differ in emitted light color are laminated.
However, when light emitting layers, which differ in emitted light color, are laminated, a problem occurs in which when the electric current density of driving electric current varies, a light emitting central position a in the light emitting layer is shifted, whereby emitted light color varies. Specifically, when the light emitting center is located near the interface of the laminated light emitting layer, the light emitting center occasionally shifts into a different color light emitting layer. In view of color rendering and color reproduction, the above shift has been a problem to overcome. An organic electroluminescent element is disclosed, which is capable of retarding the change of emitted light chromaticity by doping the organic light emitting layer, on one side adjacent to the specified organic light emitting layer, with auxiliary light emitting compounds which emit light of a color which is similar to that of the basic light emitting compounds in the above specified organic light emitting layer (for example, Patent Document 5). However, when the above method is employed, the shift of the above light emitting center is permitted only in one direction. Due to that, it becomes necessary to control the carrier conveyance of the host material to increase the restriction of materials and it becomes difficult to simultaneously realize targeted electric power efficiency and targeted light emitting life time.
In either case, it was found that by employing the above methods, when white light emission was achieved, in view of compatibility of the targeted electric power efficiency and stability of chromaticity during driving, no satisfaction was realized. Though, depending on usage, in white light emitting devices, it is commonly demanded that the resulting white chromaticity is stable during driving and for the change of driving electric current. For example, when employed as a lighting source for illumination or a backlight of a liquid crystal display, readily visible changes such that white changes toward blue or red direction significantly degrade commercial value, however high the light emitting efficiency may be. Consequently, it has been demanded to overcome the above drawbacks.                (Patent Document 1) U.S. Pat. No. 6,097,147        (Patent Document 2) Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 6-207170        (Patent Document 3) JP-A No. 2004-235168        (Patent Document 4) WO No. 2004/077886        (Patent Document 5) JP-A No. 2004-6165        (Non-Patent Document 1) M. A. Baldo et al., Nature, Volume 395, pages 151-154 (1998)        (Non-Patent Document 2) M. A. Baldo et al., Nature, Volume 403, No. 17, pages 750-753 (2000)        